Dropping off hire car to Hertz in Florence

Have you considered turning the car into the airport Hertz location? Then taking a bus into the center city--if that's your final destination in Florence.

I used a location 2 mi. from the center city, but it's since been closed.

You need to contact the rental office and find out what the required route is. You are correct in assuming that returning the rental car does not allow you to enter any ZTL. If I recall from other discussions, if you are arriving from the South, you need to drive around the center core and approach the train station from the Northwest, taking a specific one or two streets, and hope you do not make a wrong turn, or miss your turn, having to reroute.

Not sure if an app like Google Maps or Waze would help.

Go to https://www.viamichelin.com/, click on maps; enter Ponte Amerigo Vespucci Firenze. Click on the map to clear it.

Your destination is Via Borgo Ognissanti 96, the Interparking Garage Europa; second street after crossing the bridge. Note that approaching the bridge, the ZTL is to the east. Do not use google maps; stay out of dedicated bus lanes.

Regarding the first answer, the FLR complex is at Via Palagio degli Spini. There is no bus to town. You can take the shuttle to the terminal and the tram to the SM Novella station.

If you drive from Ancona, the fastest way is via Bologna (A14 then A1).

If you are indeed coming from Bologna, you will be exiting at FIRENZE NORD, and passing by the airport. So, if you don't feel comfortable driving into the city, you might as well drop the car at the Airport Rental Car Center. From there then you can take the rental shuttle van to the main terminal (5 min ride) and from there take the light rail T2 to the city center (SMN station or Unità, which is the last T2 stop, slightly closer to the Duomo).

Set your Map to make a stop here after you exit the FIRENZE NORD exit. It's the last gas station before the Airport Car Rental center
https://maps.app.goo.gl/2vXGvV8gMLpioV8T6

After you refuel the car, then head to the Car Rental center (5 min later) here:
https://maps.app.goo.gl/iL1rgCtm4kbQsQo19

Hertz downtown is now located on Via Alamanni (see Google Map link below), and I'm not sure if you can return it on Borgo Ognissanti (Garage Europa) anymore. The location below on via Alamanni has the underground garage below the office. To access this location there is no way to avoid the ZTL camera (located at the corner of Via Alamanni and Viale Fratelli Rosselli), therefore if you return it here, you must ensure that Hertz agents put your car's license plate into the City White List on the City Website to prevent incurring on a fine.
https://maps.app.goo.gl/iL1rgCtm4kbQsQo19

If Hertz still retained the rental car return also on Borgo Ognissanti (Garage Europa) then the instructions on how to avoid the cameras are below, if you prefer to return it there.
https://community.ricksteves.com/travel-forum/italy/florence-car-rental-zlt-zone

Checked re the Garage Europa, and no Hertz; too bad, because it was outside the ZTL.

Thanks for your replies, all very much appreciated. Dropping off at the airport is definitely preferable rather than running the risk of a fine.